It is known to treat a hydrocarbon flow in order to remove sulfur-comprising components and carbon dioxide. The hydrocarbon flow can, for example, be natural gas or a gas resulting from an oil refinery.
The industries for the production of oil or natural gas or for the refining of oil produce sour gases rich in CO2 and in sulfur-comprising compounds (mainly H2S) which have to be treated before being able to be released to the atmosphere.
A common process for treating these sour gases is the Claus process, in which hydrogen sulfide (H2S) is partially converted into sulfur dioxide (SO2) by combustion in the air in a furnace:2H2S+3O2→2SO2+2H2O
At the outlet of the Claus furnace, the stoichiometric proportions of H2S and of SO2 make possible the Claus reaction; this reaction takes place in several catalytic reactors:2H2S+SO23S+2H2O
To sum up, the Claus process converts hydrogen sulfide into elemental sulfur by combustion and catalytic reaction.
A known improvement to the Claus process is the replacement of air by an oxygen-rich gas (comprising between 50 mol % and 99.8 mol % of oxygen), which makes it possible to obtain flue gases more concentrated in CO2 (not diluted by atmospheric nitrogen) and to make possible the purification of these flue gases.
The traces of H2S present in the effluent stream from the Claus unit (the tail gases) can be treated by a unit known as a TGT (Tail Gas Treatment) unit, where the effluent stream from the Claus unit is treated by absorption with a solvent in order to reduce its content of sulfur-comprising components, such as CO2 and/or H2S.
At the outlet of the TGT unit, the effluent stream depleted in H2S comprises mainly CO2, and hydrogen generated during the stages of purification of the tail gases. A typical composition can be between 50 mol % and 90 mol % of CO2 and between 5 mol % and 30 mol % of hydrogen.
The flue gases from a conventional Claus unit (with combustion in the air) can be treated by units for separation by amines (of MEA or MDEA type). These units make it possible to obtain a pure CO2 stream (>99% of CO2 in general).
The disadvantages of the units for absorption by solution of amines are the high operating costs, by virtue of the strong need for steam for the regeneration of a CO2-laden solvent and the gradual deterioration in the solvent, which requires a constant supply. The capital cost is also high. Beyond the economic aspect, the risks of emissions and of toxicity of these solvents require very restrictive operating procedures.
An alternative solution is the treatment of the effluent streams from the Claus/TGT units fed with oxygen by a membrane separation unit where a stream rich in CO2 is produced in the residue (high-pressure side of the membrane) and a stream depleted in CO2 is produced in the permeate (low-pressure side of the membrane), this permeate being recycled to the Claus furnace in order to dilute the reaction gases and to control the temperature within the furnace. This is because the use of pure oxygen in the place of air requires the addition of a gas ballast (role of nitrogen for air) which will absorb the energy of the combustion reaction and limit the rise in the temperature within the furnace.
The disadvantages of this solution are as follows:                presence of H2 (from 25 to 35 mol %) in the permeate: the recycling of the permeate to the combustion furnace generates additional exothermicity by the combustion of the hydrogen, whereas the role of the ballast is to capture a portion of the energy of the oxidation reaction of the hydrogen sulfide,        the recycling of the permeate (comprising from 65 to 75 mol % of CO2) has the effect of enlarging the Claus unit and all the downstream units,        the CO2 produced has a purity limited to 98%,        the recovery yield is of the order of 80% without considering the recycling of the permeate; by recycling the permeate, the CO2 recovery can reach up to 100% results in the enlarging of the Claus/TGT unit and of all the downstream units (compression, drying, membranes),        it is difficult to recover in value economically the H2 present in the flue gases (the permeate is too depleted in hydrogen and furthermore at low pressure).        